The present application relates generally to digital communications systems, and more specifically to improved systems and methods of detecting the presence of a transmitted message at a receiver in a noisy environment.
In recent years, the orthogonal frequency division multiplexing (OFDM) technique has been increasingly employed in digital communications systems due to its resilience to noise and multi-path interference. A conventional OFDM communications system includes an OFDM transmitter and an OFDM receiver. The OFDM transmitter is operative to convert a digital signal to be transmitted to a plurality of sub-signals, each of which corresponds to a different sub-carrier frequency. The OFDM transmitter is further operative to map the data contained in each sub-signal to corresponding amplitude and phase information, to convert the mapped sub-signals from the frequency domain to the time domain using the inverse fast Fourier transform (IFFT) algorithm, and to convert the digital sub-signals to analog form for subsequent transmission at the various sub-carrier frequencies. The OFDM receiver is operative to receive transmitted sub-signals, to convert the analog sub-signals to digital form, to convert the received sub-signals from the time domain to the frequency domain using the fast Fourier transform (FFT) algorithm, and to subject the received sub-signals to phase detection processing to detect the signals' presence and to extract the data contained therein.
Although the conventional OFDM communications system transmitting signals at multiple sub-carrier frequencies generally has increased resiliency to noise and multi-path interference, the system has drawbacks in that the detection of the transmitted signals at the OFDM receiver can be problematic. For example, each signal to be transmitted typically comprises at least one packet including multiple frames, each of which contains multiple waveform samples. To detect a transmitted signal, the OFDM receiver generally samples each transmitted signal at a given sampling frequency. Accordingly, if the sampling of the transmitted signals at the OFDM receiver is synchronized with the timing of the OFDM transmitter and the samples corresponding to the start of the respective frames are known, then the data contained in the signals can be detected by simply reversing the mapping of the signal phases to their corresponding data values.
In conventional OFDM communications systems, however, the OFDM receiver is typically remote from the OFDM transmitter, and the transmitted signals generally arrive at the remote receiver asynchronously. As a result, not only may the OFDM receiver have difficulty determining which ones of the received samples correspond to the start of each transmitted frame, but it also may have difficulty detecting the mere presence of transmitted signals, especially in noisy environments.
One way of detecting the presence of transmitted signals at a remote OFDM receiver is to determine the presence (or absence) of one or more of the sub-carrier signals. However, this approach has drawbacks because to determine whether or not a sub-carrier is present, it is usually necessary to compare the signal power at frequencies different from the transmitted sub-carrier frequencies. As a result, additional bandwidth must typically be allocated at the receiver to accommodate these added frequencies. Moreover, in very noisy environments, it is often difficult to discriminate between transmitted signals and noise based solely upon a consideration of the received signal power.
It would therefore be desirable to have an improved method of detecting transmitted signals in digital communications systems that avoids the drawbacks of the above-described conventional communications systems.